The doepfer midi to sync box is about 120-140 usd new, but you can find other converters for under 50 dollars on ebay regularly. roland, korg, and a few other major manufacturers made converters that sell between 20 and several hundred used depending on the features. There area also some kits available from different sources.

I'm sure there are plenty of threads on here about different models. I doubt it's worth it with so many basic options already available in the market.

I would perhaps be interested in a basic, inexpensive one that doesn't require an external power source. Especially if it can take one midi in and turn it into 2 or 3 sync outs. Eg- 303, 808, and 606 all synced from one Midi out; Depending on how tight the sync is I guess. Might not be possible/ practical with tight timing...

Well, I built one This box covers exactly what I needed, just a simple, passive, and inline MIDI to sync24 generator, although it wouldn't be difficult to build a sync24 to MIDI box either, or one that does both. This should also theoretically power multiple sync24 or MIDI devices with a slight redesign. If you guys are interested, I could probably do $40 for one of these. Here's what it looks like:

And a jam of my mc-505, esx-1, juno-106, esq-1, and pro-one playing in sync with my tr-606 using the box above:

I'm guessing that if you feed it anything other than pure clock, it sh*ts the bed. To do it properly you'd need to extract clock pulses, start, stop and continue from your MIDI - which would either require more circuitry, or a processor of some kind. I can't make out what chips you've got in there... so if you are using something programmable already, ignore me!